In recent years, power modules are used for controlling heavy-current for electric vehicles, electric railcars and machine tools. In conventional power modules, a metal/ceramic insulating substrate is fixed to one side of a metal plate or compound material called base plate by soldering or the like, and semiconductor chips are fixed to a metal circuit plate of the ceramic insulating substrate by soldering. On the other side (reverse face) of the base plate, a radiating fin or cooling jacket of a metal is mounted via a thermal conduction grease by means of screws or the like.
Such a metal/ceramic bonding substrate is easy to warp after, metal plates (a metal circuit plate and a base plate) are bonded to both sides of a ceramic substrate thereof since the metal plates generally have different thicknesses. In addition, the soldering of semiconductor chips to the metal/ceramic bonding substrate is carried out by heating, so that the metal circuit plate and the base plate are easy to warp due to the difference in coefficient of thermal expansion between the bonded members during soldering. Moreover, heat generated from the semiconductor chips passes through the metal/ceramic insulating substrate, solder and base plate to be radiated to air or cooling water by the radiating fin or cooling jacket. Therefore, if the base plate warps during soldering, when the radiating fin or cooling jacket is mounted on the base plate, the clearance therebetween increases, so that the heat sink characteristic of the base plate extremely deteriorates. In addition, even if a metal/ceramic bonding substrate substantially having no warpage is obtained, the warpage of the substrate increases if thermal shocks are repeatedly applied thereto.
In order to solve such problems, there is proposed a circuit board wherein a circuit plate of aluminum or an aluminum alloy is formed on one side of a multilayer structure having a plurality of aluminum nitride substrates bonded to each other via aluminum plates or aluminum alloy plates and wherein a radiating plate is formed on the other side of the multilayer structure (see, e.g., Japanese Patent Laid-Open No. 2001-7465). This circuit board is produced by bonding the circuit plate of aluminum or the aluminum alloy and the radiating plate, by means of a bonding material (a brazing filler metal), to the multilayer structure which is manufactured by bonding the plurality of aluminum nitride substrates and the aluminum or aluminum alloy plates to each other by means of a bonding material (a brazing filler metal).
There is also proposed an insulating circuit board formed by a laminated structure wherein a circuit plate is bonded to one side of a first ceramic plate, wherein a thermal diffusing plate, a second ceramic plate and a radiating plate are bonded to the other side of the first ceramic plate, wherein the circuit plate, the thermal diffusing plate and the radiating plate are made of copper, an copper alloy, aluminum or an aluminum alloy, and wherein the end portion of the thermal diffusing plate is integrated with the end portion of the radiating plate so that the second ceramic plate is completely covered with the thermal diffusing plate and radiating plate (see, e.g., Japanese Patent Laid-Open No. 2003-86747). This insulating circuit board is produced by setting the first and second ceramic plates in a mold, injecting a molten metal of aluminum or an aluminum alloy in the mold at a high pressure for forming the circuit plate, the thermal diffusing plate and the radiating plate, and then, cooling and solidifying the molten metal.
However, the radiating performance of the circuit board disclosed in Japanese Patent Laid-Open No. 2001-7465 is deteriorated by the aluminum nitride substrate since the thermal conductivity thereof is lower than that of the radiating plate (metal base plate) of copper, aluminum or an alloy thereof although the aluminum nitride substrate has a high thermal conductivity (170 W/mK) as a ceramic substrate. Even if a ceramic substrate having a thermal conductivity equal to or higher than that of the metal base plate can be used, such a ceramic substrate is very expensive, so that it is difficult to industrially mass-produce circuit boards using such very expensive ceramic substrates. In addition, in order to bond the plurality of aluminum nitride substrates and the aluminum or aluminum alloy plates to each other to produce the multilayer structure and in order to bond the circuit plate of aluminum or the aluminum alloy and the radiating plate to the multilayer structure, it is required to apply very high pressures using the bonding material (brazing filler metal), and it is required to carry out a laminating step for producing the multilayer structure, so that the producing costs are high.
Similar to the circuit board disclosed in Japanese Patent Laid-Open No. 2001-7465, the radiating performance of the insulating circuit board disclosed in Japanese Patent Laid-Open No. 2003-86747 is deteriorated by the second ceramic plate since the thermal conductivity of the second ceramic plate covered with the thermal diffusing plate and radiating plate is lower than those of the thermal diffusing plate and radiating plate. In addition, when the insulating circuit board disclosed in Japanese Patent Laid-Open No. 2003-86747 is produced, it is difficult to inject the molten metal of aluminum or the aluminum alloy into the mold at a high pressure while the ceramic plate is held at a predetermined position in the mold. Therefore, there are some cases where it is not possible to bond the ceramic plates at appropriate positions, so that it is difficult to completely cover the second ceramic plate with the thermal diffusing plate and the radiating plate to precisely position the second ceramic plate. For that reason, it is difficult to control the warpage of the insulating circuit board, so that there is the possibility that the reliability thereof may be deteriorated by the variation in warpage thereof.